a) Field of the Invention
The present invention relates to an amplifier formed by a transistor. More particularly, the invention pertains to a circuit for preventing oversaturation of a transistor which forms a current amplifier.
b) Background Art
Known current amplifiers employing transistors, generally, have circuit constructions such as are shown in FIGS. 9 and 10.
The current amplifier depicted in FIG. 9 uses a PNP type transistor Q.sub.1, which has an emitter connected to a power supply Vcc. A load L is connected between a collector of transistor Q.sub.1 and ground. A base of the transistor Q.sub.1 is connected to an input terminal T, so that a base current Ib.sub.1 flows from the base of the transistor Q.sub.1 to the input terminal T. Thus, the transistor Q.sub.1 supplies the load L with a collector current Ic obtained by multiplying the base current Ib.sub.1 by a current amplification factor .beta..
The current amplifier shown in FIG. 10 is of a type employing an NPN type transistor Q.sub.11, which has its emitter grounded, and the load L is connected between a collector of the transistor Q.sub.11 and the power supply Vcc. A base of the transistor Q.sub.11 is connected to the input terminal T, from which a base current Ib.sub.11 flows to the base of the transistor Q.sub.11. The transistor Q.sub.11 supplies the load L with the collector current Ic obtained by multiplying the base current Ib.sub.11 by the current amplification factor .beta..
FIG. 11 illustrates relationships among the collector current Ic, the base current Ib and a collector-emitter voltage Vce, which are supplied by the transistor Q.sub.1 or Q.sub.11 forming the current amplifier. In the FIG. 11, the abscissa and ordinate are drawn to represent the collector-emitter voltage Vce and collector current Ic, respectively, and a particular relation is shown between those voltage Vce and current Ic in each of the transistors Q.sub.1 and Q.sub.11, in accordance with the variation of base current Ib flowing in the transistor. Further, as evident from FIG. 11, if the collector emitter voltage Vce is somewhat higher, the collector current Ic is substantially proportional to the base current Ib, but if the collector-emitter voltage Vcc becomes lower, then the same voltage Vcc is proportional to the collector current Ic in the region where a certain amount of the base current Ib flows. At this point, the voltage Vcc is a saturation voltage Vces. In the case where the transistor Q .sub.1 or Q.sub.11 is saturated by supplying a sufficient amount of base current Ib, the collector current Ic, the saturation voltage Vces and ambient temperature Ta bear such the relationships as shown in FIG. 12.
In FIG. 12, there are depicted the relationships between the collector current Ic and the saturation voltage Vces, with ambient temperature Ta varied, wherein the abscissa represents the collector current Ic and the ordinate the saturation voltage Vces. As is apparent from FIG. 12, the saturation voltage Vces is in direct proportion to the collector current Ic and has a positive temperature characteristic, and hence the resistance between the collector and emitter of the transistor in the saturated state is equivalent to a resistor Rces which has a positive temperature coefficient.
In the current amplifiers shown in FIGS. 9 and 10, when the transistors Q.sub.1 and Q.sub.11 are in the saturated state, a further increase of the base current Ib will make the transistors Q.sub.1 and Q.sub.11 oversaturated, rendering unstable their operations of current amplifiers. Also, the current amplifiers will similarly become unstable in operation when the internal impedance of the load L increases abruptly or a voltage is induced in the load L by an external factor.
Therefore, oversaturation of transistors must be prevented. This could be achieved by detecting the moment when the transistor is saturated, through utilization of its saturated voltage and then keeping the base current Ib from leaving or entering the base of the transistor in excess of the base current Ib which left (in the case of the PNP type transistor Q.sub.1) or entered (in the case of the NPN type transistor Q.sub.11) the base of the transistor at the moment of its saturation.
It is not easy, however, to detect the saturated state of the transistor by the saturation voltage Vces. The reason for this is that the saturation voltage Vces is influenced by the collector current Ic and ambient temperature Ta as will be seen from FIG. 12 and undergoes substantial changes according to various conditions.